Lithographic apparatus and a method for determining a polarization property

ABSTRACT

A method and apparatus are provided for measuring the apodization of projection optics for use in a lithographic apparatus, the projection optics having an object plane where, in use, a reticle is placed, a pupil plane, and an image plane where, in use, a wafer is placed. The method includes placing one or more Appropriate apertures in said object plane for creating a substantially uniform light distribution, illuminating the or each aperture and measuring the intensity distribution at a plane which is conjugate to the pupil plane in order to calculate the apodization of the projection optics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and method for apodizationmeasurement in a lithographic apparatus.

2. Background of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, which is alternatively referredto as a mask or a reticle, may be used to generate a circuit patterncorresponding to an individual layer of the IC, and this pattern can beimaged onto a target portion (e.g. comprising part of, one or severaldies) on a substrate (e.g. a silicon wafer) that has a layer ofradiation-sensitive material (resist). In general, a single substratewill contain a network of adjacent target portions that are successivelyexposed. Known lithographic apparatus include so-called steppers, inwhich each target portion is irradiated by exposing an entire patternonto the target portion at once, and so-called scanners, in which eachtarget portion is irradiated by scanning the pattern through theprojection beam in a given direction (the “scanning”-direction) whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection.

We first discuss here some background concerning lens apodization.

Lens light non-uniformity is in general characterized in the imageplane. Very tight specifications are used in order to limitCD-variations (critical dimension variations) through the image field.When light non-uniformity appears in the pupil plane, (e.g. over thevarious angles at image level), CD-variations through pitch areexpected. Light non-uniformity in the pupil is often referred to asapodization. In this sense, apodization describes the amplitude part ofthe pupil-transmission function (where aberrations describe the phasepart of the pupil transmission function). A uniform pupil transmissionis often assumed, but this is generally not the case.

Below we give a definition of lens apodization and its relation to pupilmeasurement. Next we give an overview of the different types ofapodization and their impact on imaging.

Mathematically imaging can be described by two Fourier transforms: onefrom the object plane to the pupil plane and one from the pupil plane tothe image plane. Prior to the second Fourier transform, the pupildistribution must be multiplied by the OTF (optical transfer function)of the imaging system.${I\quad\left( \overset{\rightarrow}{x} \right)} = {\int_{\overset{\rightarrow}{\rho}}{{OTF}\quad{\left( {\overset{\rightarrow}{x},\overset{\rightarrow}{\rho}} \right) \cdot \quad\left\lbrack {\int_{\overset{\rightarrow}{x}}{O\quad{\left( {\overset{\rightarrow}{x}}^{\prime} \right) \cdot {\mathbb{e}}^{2 \cdot \pi \cdot {\mathbb{i}} \cdot {\overset{\rightarrow}{x}}^{\prime} \cdot \overset{\rightarrow}{\rho}}}}} \right\rbrack \cdot {\mathbb{e}}^{{- 2} \cdot \pi \cdot {\mathbb{i}} \cdot {\overset{\rightarrow}{x}}^{\prime} \cdot \overset{\rightarrow}{\rho}}}}}$

Here, I and O equal the E-fields in the image and object planes asfunction of the field co-ordinates {right arrow over (x)}, and pupilco-ordinates {right arrow over (ρ)}. In order to obtain the imageintensity, the amplitude of the E-field needs to be squared.

The OTF can be split into a phase term W (describing the aberrations)${{OTF}\quad\left( {\overset{\rightarrow}{x},\overset{\rightarrow}{\rho}} \right)} = {A\quad{\left( {\overset{\rightarrow}{x},\overset{\rightarrow}{\rho}} \right) \cdot {\mathbb{e}}^{{{- 2} \cdot \pi \cdot {\mathbb{i}} \cdot W}\quad{({{\overset{\rightarrow}{x}}^{\prime} \cdot \overset{\rightarrow}{\rho}})}}}}$and an apodization term A (describing the apodization). Both are afunction or the pupil co-ordinate {right arrow over (ρ)}, and vary overthe field with field co-ordinates {right arrow over (x)}.

Next we turn to an apodization definition. Apodization is defined as theangular lens transmission. The apodization function A(r) can bedecomposed into several classes of functions. In analogy withaberrations Zernike polynomials Z_(n)({right arrow over (ρ)}) and thecorresponding Zernike coefficients z_(n)({right arrow over (x)}) are anappropriate decomposition.${A\quad\left( {\overset{\rightarrow}{x},\overset{\rightarrow}{\rho}} \right)} = {\sum\limits_{n}{{z_{n}\left( \overset{\rightarrow}{x} \right)} \cdot {Z_{n}\left( \overset{\rightarrow}{\rho} \right)}}}$

These Zernike coefficients have no dimension, representing thetransmission of the corresponding Zernike polynomial at the pupil-edge.

In line with the assumption of uniform lens transmission, lensapodization is currently typically not measured directly. Variouscomponents of it are part of or influence other measurements. E.g. theuniform pupil transmission Z₁(x) is part of the uniformity measurement.

Current pupil measurements at wafer-level measure a combination of theillumination angular intensity and the lens apodization. No separationis made between lens and illuminator effects. This can be dangeroussince both induce different imaging effects.

To understand the impact on imaging, one has to look at the diffractionpattern of the object. In general, large objects have small diffractionpatterns, while small objects have large diffraction patterns.Diffraction patterns are (in general) symmetric; the amplitude of thepositive diffraction orders is equal to the amplitude of the negativediffraction orders.

Odd apodizations affect the telecentricity of the light at wafer level.Telecentricity equals the mean pointing (i.e. mean direction) of theimaging beam. In this way it affects overlay as a function of focus.That is to say, for odd apodizations more light reaches a given point onthe wafer (image plane) from one side than from the other, with theresult that if the wafer is moved up or down with respect to the imageplane (thus defocusing the exposed image) the image effectively moveshorizontally.

Even apodizations (for which the light intensity reaching a given pointon the wafer (image plane) is symmetrical about a line perpendicular tothe wafer) affect the optimal exposure dose as a function of structuredensity (pitch) and orientation. Thus it results in CD variationsthrough pitch; lines of different pitch require a different exposuredose to be printed at the same size.

As a special case, astigmatic apodization introduces an energydifference between horizontal and vertical lines.

Apodization appears within the lens pupil. Thus it can only be measuredwith a pupil sensor. One can use e.g. a pinhole sensor scanning at largedefocus, or a parallel detector array, conjugate with the pupil plane.All such sensors contain optical interfaces and/or may not be perfectlyconjugate with the pupil and thus need to be corrected for geometricaleffects as well as angular transmission effects (Fresnel coefficients).

In an optical context conjugate points are points on the optical axis ofan optical system, such as a lens or mirror, so positioned that lightemitted from either point will be focused at the other, e.g. object andimage points. By extension, conjugate planes are planes normal to theoptical axis of an optical system so positioned that light emitted frompoints in either plane will be focused at the other, e.g. object andimage planes.

Next one has to realise a known pupil filling at object level. This canbe either realised by calibrating or measuring a well-defined intensitydistribution, or by use of a theoretically known intensity distributionsuch as given by a perfect pinhole, with a diameter small with respectto the wavelength used.

US 2002/0001088 (corresponding to WO 01/63233 in the name of Carl Zeiss)describes an apparatus for wavefront detection which includes an opticalsystem for transforming a wavefront, a diffraction grating through whichthe wavefront passes, and a spatially resolving detector following thediffraction grating. FIG. 1 of the Zeiss patent shows the layout of theapparatus, in which light from a source 43 passes through a movablelight guide 29, a perforated mask 8, lenses 13 and 15, and a moveablediffraction grating 11 before reaching a detector 19 which comprises asensor surface 20. The apparatus of the Zeiss patent can be used for themeasurement of lens aberration. The function of the grating 11 is toconvert phase effects (caused by aberrations) to amplitude effects,which are then measured by the detector 19. In the Zeiss patent scanningis also required because it is necessary to separate x and y components.The scanning allows one component to be averaged out so that the othercan be measured.

SUMMARY OF THE INVENTION

According to the invention, there is provided a method of measuring theapodization of projection optics for use in a lithographic apparatus,the projection optics having an object plane where, in use, a reticle isplaced, a pupil plane, and an image plane where, in use, a wafer isplaced, the method comprising the steps of:

placing one or more Appropriate apertures in said object plane forcreating a substantially uniform light distribution;

illuminating the or each aperture; and

measuring the intensity distribution at a plane which is conjugate tothe pupil plane in order to calculate the apodization of the projectionoptics.

In certain embodiments, said plane which is conjugate to the pupil planecorresponds to a plane which is out-of-focus with respect to the imageplane such that a far field imaging condition applies with respect tosaid pupil plane.

Said apertures may be an array of apertures.

In certain embodiments, the apertures of said array are arranged in aquasi-random or random manner.

The or each aperture may have a diameter smaller than the wavelength ofradiation used, a diameter approximately equal to the wavelength of theradiation used, or a diameter greater than the wavelength of theradiation used.

The method may comprise the further step of performing a calibration toestablish the intensity profile of light emitted from the aperture orapertures.

The calibration may be performed off-line.

The method may comprise the further step of placing a diffuser over theor each aperture to diffuse the light reaching the or each aperture.

The invention also provides apparatus for measuring the apodization ofprojection optics for use in a lithographic apparatus, the projectionoptics having an object plane where, in use, a reticle is placed, apupil plane, and an image plane where, in use, a wafer is placed, theapparatus comprising:

one or more Appropriate apertures located in said object plane forcreating a substantially uniform light distribution;

illuminator for illuminating the or each aperture; and

light measuring sensor for measuring the intensity distribution at aplane which is conjugate to the pupil plane in order to calculate theapodization of the projection optics.

In certain embodiments, said plane which is conjugate to the pupil planecorresponds to a plane which is out-of-focus with respect to the imageplane such that a far field imaging condition applies with respect tosaid pupil plane.

The apertures of the apparatus may be an array of apertures, and may bearranged in a quasi-random or random manner.

The or each aperture may have a diameter smaller than the wavelength ofradiation used, a diameter approximately equal to the wavelength of theradiation used, or a diameter greater than the wavelength of theradiation used.

The intensity profile of light emitted from the aperture or aperturesmay be calibrated.

The apparatus may be provided with a diffuser over the or each apertureto diffuse the light reaching the or each aperture.

The invention also provides a lithographic apparatus comprising:

an illumination system for providing a projection beam of radiation;

a support structure for supporting a patterning device, the patterningdevice serving to impart the projection beam with a pattern in itscross-section;

a substrate table for holding a substrate; and

projection optics for projecting the patterned beam onto a targetportion of the substrate,

said lithographic apparatus further comprising

a system for measuring the apodization of the projection optics, theprojection optics having an object plane where, in use, a reticle isplaced, a pupil plane, and an image plane where, in use, said substrateis placed, the apparatus comprising:

one or more Appropriate apertures located in said object plane forcreating a substantially uniform light distribution; and

a light measuring device for measuring the intensity distribution at aplane which is conjugate to the pupil plane in order to calculate theapodization of the projection optics.

In certain embodiments, said plane which is conjugate to the pupil planecorresponds to a plane which is out-of-focus with respect to the imageplane such that a far field imaging condition applies with respect tosaid pupil plane.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion”, respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of5-20 nm), as well as particle beams, such as ion beams or electronbeams.

The term “patterning device” used herein should be broadly interpretedas referring to devices that can be used to impart a projection beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the projection beam may not exactly correspond to thedesired pattern in the target portion of the substrate. Generally, thepattern imparted to the projection beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit.

Patterning devices may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a way depending on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support can beusing mechanical clamping, vacuum, or other clamping techniques, forexample electrostatic clamping under vacuum conditions. The supportstructure may be a frame or a table, for example, which may be fixed ormovable as required and which may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device”.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “lens” herein may be considered assynonymous with the more general term “projection system”.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation, and such components may also be referred to below,collectively or singularly, as a “lens”.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index. e.g.water, so as to fill a space between the final element of the projectionsystem and the substrate. Immersion liquids may also be applied to otherspaces in the lithographic apparatus, for example, between the mask andthe first element of the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 shows a lithographic apparatus comprising a projection lens, theapodization of which can be measured using the present invention;

FIG. 2 shows an overview of the measurement system of the presentinvention;

FIG. 3 shows the use of an array of pinholes; and

FIGS. 4 and 5 shows the use of a diffuser above the pinhole or array ofpinholes.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus in which theinvention may be employed. The apparatus comprises:

an illumination system (illuminator) IL for providing a projection beamPB of radiation (e.g. DUV or EUV radiation).

a first support structure (e.g. a mask table) MT for supporting apatterning device (e.g. a mask) MA and connected to first positioner PMfor accurately positioning the patterning device with respect to itemPL;

a substrate table (e.g. a wafer table) WT for holding a substrate (e.g.a resist-coated wafer) W and connected to second positioning device PWfor accurately positioning the substrate with respect to item PL; and

a projection system (e.g. a refractive projection lens) PL for imaging apattern imparted to the projection beam PB by patterning device MA ontoa target portion C (e.g. comprising one or more dies) of the substrateW.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above).

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising for examplesuitable directing mirrors and/or a beam expander. In other cases thesource may be integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise adjustable elements AM for adjusting theangular intensity distribution of the beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator ILgenerally comprises various other components, such as an integrator INand a condenser CO. The illuminator provides a conditioned beam ofradiation, referred to as the projection beam PB, having a desireduniformity and intensity distribution in its cross-section.

The projection beam PB is incident on the mask MA, which is held on themask table MT. Having traversed the mask MA, the projection beam PBpasses through the lens PL, which focuses the beam onto a target portionC of the substrate W. With the aid of the second positioner PW andposition sensor IF (e.g. an interferometric device), the substrate tableWT can be moved accurately, e.g. so as to position different targetportions C in the path of the beam PB. Similarly, the first positionerPM and another position sensor (which is not explicitly depicted inFIG. 1) can be used to accurately position the mask MA with respect tothe path of the beam PB, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the object tables MTand WT will be realized with the aid of a long-stroke module (coarsepositioning) and a short-stroke module (fine positioning), which formpart of the positioner PM and PW. However, in the case of a stepper (asopposed to a scanner) the mask table MT may be connected to a shortstroke actuator only, or may be fixed. Mask MA and substrate W may bealigned using mask alignment marks M1, M2 and substrate alignment marksP1, P2.

The depicted apparatus can be used in the following preferred modes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theprojection beam is projected onto a target portion C at once (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the projection beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the projection beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

We now turn to measurement of apodization.

The angular distribution of the light exiting the Projection Lens can bemeasured by a sensor such as the TIS or ILIAS. However, to measure thetransmission of the Projection Lens a source is needed with a uniform orknown angular distribution of the light. The Illuminator cannot providesuch a uniform illumination.

In state-of-the-art lithographic apparatus several in-situ techniquesare used to measure the pupil intensity distribution, such as a scanningpinhole out-of-focus, or a parallel detector array, effectivelyconjugate with the pupil plane.

Measurement and control of the pupil shape is becoming critical forscanner performance at low k1-factors, where k1 is the ratio between CD(Critical Dimension) and wavelength divided by NA (Numerical Aperture).

Looking at the optical system of FIG. 1 rigorously, in both techniquesmentioned above the measured pupil intensity distribution is the productof the projection lens transmission as a function of the pupil planecoordinates (referred to in this specification as apodization) and theilluminator intensity profile.

In terms of imaging, these two components have a distinctive effect,since the lens transmission function is imaged coherently, while theilluminator profile is imaged incoherently. The present measurementtechniques however, cannot distinguish between these contributions.

For existing systems, to a good approximation the lens transmission canbe assumed to be uniform over the entire pupil. In the near future,however, ultra-high NA systems will require explicitly separatedmeasurements of illuminator profile and apodization.

The above-mentioned explicit separation of illuminator profile andapodization can be accomplished straightforwardly by realising anintensity distribution at object level (see pinhole 2 in FIG. 2) with aknown or calibrated angular intensity distribution. This is outlined inmore detail below.

FIG. 2 shows an overview of the measurement system, which is used formeasuring the apodization of the projection lens PL of the lithographicapparatus of FIG. 1.

FIG. 2 shows the incoming beam 1 from an illuminator, a pinhole 2 atreticle level, a uniform spherical wavefront 3 emerging from pinhole 2,the pupil plane 4, the image plane 5, and a photodiode array 6 whichacts as a camera for measuring light intensity. The projection lens PLof FIG. 1 is represented in FIG. 2 schematically as projection optics 8including a first lens 10 and a second lens 12. The pupil plane 4 iswhere the apodization of the projection optics 8 is assumed to occurmathematically. The image plane 5 corresponds with the plane of thewafer W in FIG. 1. The pinhole 2 can be realised e.g. by creating anopen area with high transmittance in a highly absorbing chrome ordielectric layer; this open area should have dimensions well below thewavelength used to act as a true point source. Such a point source hasan intensity distribution which is effectively uniform over the range ofsolid angles present within the lens pupil.

As shown in FIG. 3, the pinhole 2 may be replaced by an extended arrayof small ‘true point sources’ 2 a (by which we mean sources each havinga diameter smaller than the wavelength used), where each point source 2a has a uniform intensity distribution (as a result of diffraction atthe point source). By extending the array of sources the loss oftransmission due to the very small open area per point source is(partly) compensated for. Because these point sources 2 a add upincoherently for a detection device which is effectively conjugate tothe pupil plane, in principle no resolution is sacrificed by extendingthe array.

Transmission can be enhanced by using an n×n array of pinholes, whichincreases the effective transmission by a factor of n×n. The exactpinhole shape does not matter, just the critical dimension (i.e.diameter) which should be below the wavelength. The array layout is alsonot critical, but the structures should be close to each other (within˜0.1 mm) to avoid blurring.

A second embodiment, illustrated by FIG. 4, is to boost the effectivetransmission of the above-mentioned ‘true point sources’ 2 a bycombining an array of sufficiently small open areas with a strongdiffuser 14 to further equalise the intensity distribution over allsolid angles within the pupil plane 4. In this way, the dimensions ofthe open area per ‘point source’ can be increased as illustrated at theleft of FIG. 4 where the increase in diameter from d1 to d2 is shown. InFIG. 4, d1 represents a true point source (diameter smaller thanwavelength), whereas d2 represents a “weak point source” (diameter ofthe same order as the wavelength) which needs the use of the strongdiffuser 14 to ensure a uniform intensity distribution. The use of pointsources of larger diameter increases the overall intensity and alsorelaxes the reticle lithography requirements in conformance tostate-of-the-art reticle lithography (200-300 nm). This greatlyfacilitates manufacturing actual Source Modules (being the modules wherethe effective source is located) at minimal Cost-of-Goods.

A third embodiment, shown in FIG. 5, is to use a ‘weak point source’,with dimensions larger than the used wavelength (represented by theaperture on the left of FIG. 5 having diameter d2), and to calibrate theintensity distribution against that of a ‘true point source’ (with thedrawback of low transmission), for example on a system which has asurplus of intensity for pupil measurements (e.g. a high-throughput 248nm lithographic apparatus). In this embodiment the point source diameter(d2) is increased so much compared to that of a true point source (d1)that its intensity distribution needs to be calibrated even when usedwith the strong diffuser 14. The resulting calibration data can beconnected to the now-calibrated Source Module by various methods.

The calibration process is basically an (off-line) comparison of theobject distribution over all angles, as resulting from this “weak pointsource”, against the essentially perfect distribution of a “true pointsource”. In such a calibration a number of drawbacks of in-lineapplications can be avoided, in particular the lack of intensity, timingconstraints and perhaps also the difficulty of creating a true pointsource in the available volume.

Using the ‘weak point source’ of FIG. 5, the overall intensity isboosted significantly, but at the expense of a loss of the intrinsicintensity uniformity of a ‘true point source’. This drawback is solvedby the (once-only) calibration.

Alternatively, this calibration of the intensity distribution at objectlevel of a given Source Module can be done outside of the lithographicapparatus, on a calibrated measurement tool or by comparison against aperfect reference Source Module.

The third embodiment of FIG. 5 may use an array of such weak pointsources, in which case the calibration is carried out for the intensitydistribution of the whole array.

The invention also offers a further improvement to any of theembodiments described above which use an array of pinholes. This furtherimprovement comprises arranging the pinholes in a quasi-random or randomarray. Simply arranging the pinholes in a regular fashion (on some grid)may create unwanted discrete diffraction orders and thus lead to anon-uniform distribution.

However, by placing a relatively large number of pinholes on aquasi-random grid these diffraction orders are avoided. Even if aperfectly uniform distribution cannot be achieved, the distribution isknown by design, and so the transmission of the Projection Lens can bemeasured. By quasi-random we mean that the arrangement of the pinholesis not regular. The arrangement of the pinholes may for example resemblethe appearance of items which have been sprinkled at random onto asurface.

In this way, a uniform source with sufficient intensity can be createdto enable measurement of apodization of the projection lens.

The embodiments described allow measurement of the projection lensapodization, independent of the illuminator profile.

By applying the found lens apodization as a correction to pupilmeasurements, it may also be used to improve measurement of theintensity profile of the illuminator system.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practised otherwisethan as described. The description is not intended to limit theinvention.

1. A method of measuring apodization of projection optics for use in alithographic apparatus, the projection optics having an object planewhere, in use, a reticle is placed, a pupil plane, and an image planewhere, in use, a wafer is placed, the method comprising: placing one ormore apertures in said object plane for creating a substantially uniformlight distribution; illuminating the aperture; and measuring anintensity distribution at a plane which is conjugate to the pupil planein order to calculate the apodization of the projection optics.
 2. Amethod as claimed in claim 1, wherein said plane which is conjugate tothe pupil plane corresponds to a plane which is out-of-focus withrespect to the image plane such that a far field imaging conditionapplies with respect to said pupil plane.
 3. A method as claimed inclaim 1, wherein said apertures comprise an array of apertures.
 4. Amethod as claimed in claim 3, wherein the apertures of said array arearranged in a quasi-random or random manner.
 5. A method as claimed inclaim 1, wherein the or each aperture has a diameter smaller than thewavelength of radiation used.
 6. A method as claimed in claim 1, whereinthe or each aperture has a diameter approximately equal to thewavelength of the radiation used.
 7. A method as claimed in claim 1,wherein the or each aperture has a diameter greater than the wavelengthof the radiation used.
 8. A method as claimed in claim 7, which furthercomprises: performing a calibration to establish an intensity profile oflight emitted from the aperture or apertures.
 9. A method as claimed inclaim 8, wherein said calibration is performed off-line.
 10. A method asclaimed in claim 1, which further comprises: placing a diffuser over theor each aperture to diffuse the light reaching the or each aperture. 11.Apparatus for measuring apodization of projection optics for use in alithographic apparatus, the projection optics having an object planewhere, in use, a reticle is placed, a pupil plane, and an image planewhere, in use, a wafer is placed, the apparatus comprising: one or moreapertures located in said object plane for creating a substantiallyuniform light distribution; an illumination source for illuminating theor each aperture; and a sensor for measuring an intensity distributionat a plane which is conjugate to the pupil plane in order to calculatethe apodization of the projection optics.
 12. A method as claimed inclaim 11, wherein said plane which is conjugate to the pupil planecorresponds to a plane which is out-of-focus with respect to the imageplane such that a far field imaging condition applies with respect tosaid pupil plane.
 13. Apparatus as claimed in claim 11, wherein saidapertures comprise an array of apertures.
 14. Apparatus as claimed inclaim 13, wherein the apertures of said array are arranged in aquasi-random or random manner.
 15. Apparatus as claimed in claim 11,wherein the or each aperture has a diameter smaller than the wavelengthof radiation used.
 16. Apparatus as claimed in claim 11, wherein the oreach aperture has a diameter approximately equal to the wavelength ofthe radiation used.
 17. Apparatus as claimed in claim 11, wherein the oreach aperture has a diameter greater than the wavelength of theradiation used.
 18. Apparatus as claimed in claim 17, wherein theintensity profile of light emitted from the aperture or apertures iscalibrated.
 19. Apparatus as claimed in claim 11, which furthercomprises a diffuser over the or each aperture to diffuse the lightreaching the or each aperture.
 20. A lithographic apparatus comprising:an illumination system for conditioning a beam of radiation; a supportstructure for supporting a patterning device, the patterning deviceserving to impart the projection beam with a pattern in itscross-section; a substrate table for holding a substrate; projectionoptics for projecting the patterned beam onto a target portion of thesubstrate and having an object plane, where, in use, the patterningdevice is placed, a pupil plane, and an image plane where, in use, thesubstrate is placed; one or more apertures located in said object planefor creating a substantially uniform light distribution; and a sensorfor measuring the intensity distribution at a plane which is conjugateto the pupil plane in order to calculate an apodization of theprojection optics.
 21. An apparatus as claimed in claim 20, wherein saidplane which is conjugate to the pupil plane corresponds to a plane whichis out-of-focus with respect to the image plane such that a far fieldimaging condition applies with respect to said pupil plane.